1. Field of the Invention
The invention relates to apparatus for controlling SQUIDs, and more particularly, apparatus for coordinating and controlling multiple SQUIDs.
2. Description of Related Art
A superconducting quantum interference device (SQUID) is made up of a superconducting loop which is broken in at least one place by a Josephson junction. If the SQUID contains only one junction, then it is known as an RF SQUID since it must be biased with an oscillating current. If the SQUID contains two or more junctions, then it is known as a DC SQUID since it can be biased with a DC current. The present invention relates primarily to the control of DC SQUIDs, but some aspects can also be used in control systems for RF SQUIDs.
In essence, a DC SQUID is a magnetic flux-to-voltage convertor since it provides an output voltage across the junctions which varies as a function of the total magnetic flux applied to the superconducting loop. The output voltage is periodic in the applied flux, with a period of one flux quantum .PHI..sub.0. Thus by applying a fixed DC flux to the SQUID to set a quiescent voltage output level, magnetic fields producing flux in the SQUID on the order of a single flux quantum can be detected by measuring the deviation of the SQUID output voltage from the quiescent value.
SQUIDs and their control systems are described in Clarke, "Principles and Applications of SQUIDs", Proceedings of the IEEE, Vol. 77, No. 8, 1208-1223 (1989), incorporated herein by reference. As described therein, most SQUID readout schemes involve the use of a flux-locked loop (FLL) circuit. Whenever the voltage output of the SQUID deviates from a quiescent value (usually zero) due to externally applied flux, the circuit generates a feedback current which is applied to a feedback coil near the SQUID. The circuit provides whatever feedback current is required such that the flux due to the feedback coil compensates for the externally applied flux, thereby canceling out the externally applied flux and returning to zero the net flux applied to the SQUID. The voltage output of the SQUID is thus maintained at the quiescent value. The feedback current level which was needed to cancel the externally applied flux is then monitored to provide an analog output signal indicating the magnitude of the externally applied flux.
It can be seen that not only does this technique permit sensing externally applied flux levels which are significantly smaller than a single flux quantum, but since it continually drives the SQUID back to a predefined operating point on the voltage/flux curve, the technique is not limited by the periodic nature of that curve. Externally applied flux levels of many times .PHI..sub.0 can be measured as long as the FLL feedback circuitry can supply the necessary compensating current, and as long as the FLL feedback circuit responds quickly enough to prevent jumps from one period of the curve to the next. The FLL technique also provides an analog output signal which varies linearly with the externally applied flux.
To be practical, an FLL may include an integrator to integrate the deviation of the SQUID voltage output from the quiescent value. Also, any DC drift problems inherent in the FLL circuit can be ameliorated by modulating the feedback current at an RF frequency of approximately 100 kHz-500 kHz and demodulating the voltage output of the SQUID at the same frequency. If the operating point of the SQUID is set at a minimum or maximum of the voltage/flux curve, and the modulation of the feedback current is by a square wave having a magnitude which is sufficient to oscillate the feedback flux supplied to the SQUID by .+-..PHI..sub.0 /4, then the voltage output of the SQUID will alternate between a point half way up the voltage/flux curve to the left of the opreating point and a point half way up the voltage/flux curve to the right of the operating point. Both voltages are equal, so the AC component of the SQUID output voltage will be zero. A SQUID in such a state is referred to herein as exhibiting an average alternating voltage at a quiescent level of zero. When an external magnetic flux is applied, the operating point will no longer be at a maximum or minimum of the voltage/flux curve. Thus the voltage output of the SQUID will alternate between points to the left and right of the operating point whose voltages are no longer equal, and the average alternating voltage output will therefore deviate above or below the quiescent value of zero. The FLL feedback circuit can compensate for the externally applied flux and thus produce the desired analog output signal. The modulating current may either be added to the feedback current provided to the feedback coil, or it may be delivered separately to a separate modulation coil located near the SQUID. Demodulation is accomplished using a conventional synchronous demodulator or lock-in detector.
As used herein, an "FLL circuit" is considered to include the SQUID itself, an optional preamplifier coupled to the SQUID, the optional lock-in detector and the optional integrator, as well as the current feedback path and the feedback coil. An "FLL feedback circuit" is considered to include the preamplifier, lock-in detector, integrator and current path back to the feedback coil, but not the feedback coil itself nor the SQUID. An FLL circuit or FLL feedback circuit is referred to herein as a modulated FLL circuit or a modulated FLL feedback circuit if the above-described modulation and demodulation technique is used.
In a typical system, the SQUID and the various coils are provided on a SQUID probe which is adapted to be maintained at cryogenic temperatures. The FLL feedback circuit typically operates at room temperature and is coupled to the probe by a probe cable. As used herein, a circuit operates at "cryogenic temperatures" if it takes advantage of superconducting phenomena. Similarly, a circuit operates at "room temperature" if it does not employ superconducting phenomena. It is not intended that a circuit be at exactly the ambient temperature of the outside environment to be considered a "room temperature" circuit. The voltage output of the SQUID may be coupled to the input of the FLL feedback circuit via one or more impedance-matching transformers as well as an optional DC blocking capacitor and/or a resonant circuit. In different designs, different ones of these coupling components are located on the probe and held at cryogenic temperatures, or located with the FLL feedback circuit and held at room temperatures.
In Applied Physics Systems, "Model 581 DC SQUID System" (Mountain View, Calif.: 1990), there is described a SQUID system which includes a DC SQUID sensor, a cryogenic probe (which, together with the DC SQUID sensor would constitute a "SQUID probe" as the term is used herein), SQUID processor electronics, and a control/display console. The cryogenic probe comprises a rod for insertion into a cryostat, and the SQUID sensor is attached to the insertion end of the tube. A brass enclosure containing RFI filters and a connector is attached to the non-inserted end of the tube. The SQUID processor electronics component is a highly shielded and filtered enclosure which contains all of the SQUID closed-loop electronics. It is coupled to the connector at the end of the SQUID probe via a short shielded cable. The SQUID processor electronics component is too large to mount easily on top of the cryostat, and certainly too large for more than two or three to be accommodated on top of the cryostat.
The Applied Physics Systems (APS) SQUID is adequate where only a single SQUID is to be inserted into the cryostat. Applications for SQUIDs are developing, however, which require a plurality of SQUIDs in operation simultaneously. For example, a SQUID system used for sensing brain wave-generated flux signals may require several SQUID sensors each disposed within one inch from another. For these types of applications, the APS technique would not be adequate for at least two reasons. First, each channel would require its own complete set of components including its own SQUID probe, its own processor electronics component, and its own control/display console. The console outputs might also be connected to a data acquisition module of a single computer, but no provisions are made for the computer to control the operation of the SQUIDs; each console would have to be adjusted by hand.
Second, in addition to the large number of components, the APS system contains no provisions to synchronize the modulation frequencies of the various channels. Since the SQUIDs are likely to be located within close proximity to each other, the feedback flux applied to one SQUID will likely be sensed in part by another SQUID. Any differences in the modulation frequencies between SQUIDs are likely to cause undesirable beats in the readings from the various SQUIDs.
In Quantum Design, "DC SQUID Sensors, Electronics and Systems" (1990), there is described a system specifically intended for multiple-channel use. The system comprises a SQUID controller which contains one "masterboard" and, for each channel, a separate "multicard". The input/output structure for each channel includes a flexible SQUID probe/sensor for insertion into a cryostat, a flexible cable for coupling the probe to room temperature electronics, a small back shell-style micropreamp, and up to ten meters of highly shielded cable for coupling the micropreamp to the respective multicard. The micropreamp contains its own RF shielding and filtering, and the DC SQUID probe and cable contains its own RF immunity and magnetic shielding. The micropreamp works with either internal or external feedback and the SQUID probe is wired for either type of application.
The Quantum Design (QD) system implements a separate flux-locked loop arrangement for each channel, but the major portion of the FLL feedback circuitry is located in the multicard, up to ten meters distant from the SQUID. Specifically, the voltage output of the SQUID is first amplified to some extent by the micropreamp, then transmitted differentially over the heavily shielded cable to the multicard. The multicard contains the lock-in detector, integrator and feedback current path. The RF modulation frequency is applied to the feedback current path (or to a separate modulation current lead) on the multicard and transmitted through the cable to the SQUID probe. Similarly, the signal output of the micropreamp still contains the RF modulation frequency, which is not removed until the signal reaches the lock-in detector on the multicard. The transmission of RF signals through the cable limits the maximum length of the cable between the SQUID probe and the controller and reinforces the requirement that the cable be heavily shielded and that signals be transmitted differentially.
In addition, each of the multicards in the QD system also contains various channel-specific digital circuits such as D/A converters for controlling the SQUID bias current, skew current and DC offset current. They also contain digitally controlled analog switches for controlling such adjustments as the gain of the FLL and the choice of an anti-aliasing filter. Accordingly, noise from the switching of these circuits could deleteriously affect the operation of the flux locked loop. Moreover, the master board, which also contains such digital circuits as multiplexers, A/D converters, a FIFO and a microprocessor, is disposed in the same SQUID controller box as all of the multicards. Noise from these components, too, are likely to affect the operation of the flux locked loops. The QD system requires extensive internal shielding to minimize these effects, and even this shielding is imperfect due to the hardware connections that must penetrate in order to control the FLL.
The controller unit of the QD system is adapted to be controlled either from a front panel or from an external computer which is coupled to the controller via an RS232 or GPIB port. Output signals read from the probes are displayed on the front panel of the controller and/or transmitted to the external computer. The commands for controlling the SQUID controller in the QD system are described in Quantum Design, Inc., "Model 5000 DC SQUID Controller Operator's Manual" (1991), incorporated herein by reference.
In Fujimaki, "Josephson Integrated Circuits III--A Single-Chip SQUID Magnetometer", Fujitsu Sci. Tech. J., Vol 27, No. 1 (1991), pp. 59-83, there is described a multiple SQUID system in which each SQUID channel includes a digital feedback loop constructed using Josephson digital circuits on the same cryogenic chip as the SQUID. However, significant additional work is required before a system using this technique will become practical.
Yet another technique for controlling SQUIDs involves the inclusion of the SQUID in a relaxation oscillator. This technique is probably not useful for controlling multiple, closely spaced SQUIDs, however, since the SQUIDs by definition will operate at different frequencies.
Accordingly, it is an object of the present invention to provide a SQUID control system which ameliorates some or all of the above-mentioned problems.